This invention relates to the synthesis of cyclohexanone oxime by reacting cyclohexanone with hydroxylamine. Cyclohexanone oxime, as is well known, is an intermediate useful in the production of caprolactam by the Beckmann rearrangement. One of the more important uses of caprolactam is in the production of nylon type fibers.
The synthesis of cyclohexanone oxime by reacting cyclohexanone with an excess of hydroxylamine over the theoretical amount for formation of cyclohexanone oxime has been suggested. The excess of hydroxylamine assures complete utilization of the cyclohexanone which otherwise tends to contaminate the oxime product. Hydroxylamine sulfate solution produced by the hydrolysis of hydroxylamine disulfonate has been suggested as the source of hydroxylamine employed in such synthesis.
Canadian Pat. No. 651,447, granted Oct. 30, 1962, disclosed a process for recovering hydroxylamine values from aqueous ammonium sulfate brine by extracting the brine at a pH of about 2.5 or higher with cyclohexanone in a mol ratio of at least about 5 to 1, preferably between 7 and 1 and about 10 to 1, at a temperature of from 40.degree. to 70.degree. C. and recovering an organic distillate consisting essentially of cyclohexanone from the brine raffinate by steam distilling off at least about 1%, but not more than about 4%, by weight of the raffinate. By hydroxylamine values is meant hydroxylamine sulfonate or other forms of hydrolyzable hydroxylamine derivatives present in the aqueous ammonium sulfate brine.
The oximation, as described, including the use of hydroxylamine sulfate liquor produced by boiling the disulfonate for about one-half hour as the source of the hydroxylamine, invariably results in some lactam formation during the oximation and also in the formation of tars which contaminate the oxime product. While the reason for tar formation is not fully known, it is believed in part at least to be due to reaction between lactam and hydroxylamine monosulfonate extracted from the ammonium sulfate brine and recycled through the oximation reaction zone. Thus the process as proposed not only results in a loss of reactants due to the formation of lactam, but also results in an oxime product contaminated with undesirable tars.
More recently, U.S. Pat. No. 3,070,627, granted Dec. 25, 1962, disclosed a process of synthesizing cyclohexanone oxime in which during the oximation of the cyclohexanone, the formation of lactam is minimized and in which, notwithstanding the recycling through the oximation reaction zone of the cyclohexanone extractant employed to recovery hydroxylamine values including the cyclohexanone oxime content of the aqueous ammonium sulfate brine, the formation of tars which tend to contaminate the oxime product is also minimized if not completely prevented.
In accordance with this patent, hydroxylamine disulfonate produced as is well known by the reaction of ammonium nitrate, sulfur dioxide and ammonia is hydrolyzed to convert at least 97% of the ammonium hydroxylamine monosulfonate produced when the disulfonate is completely hydrolyzed to a hydroxylamine reactant containing hydroxylamine sulfate, and/or hydroxylamine acid sulfate, ammonium sulfate and/or ammonium bisulfate. The resultant hydroxylamine reactant is introduced into the oximation zone; into this zone is also fed (a) the cyclohexanone extract from the extraction of the ammonium sulfate brine, which cyclohexanone extract contains hydroxylamine values removed from the brine, (b) the organic distillate consisting essentially of cyclohexanone removed from the brine raffinate, and (c) additional cyclohexanone if needed, to thus provide in the oximator a reaction mixture containing at least 5, preferably from about 5 to 15, mol percent of hydroxylamine over and above the theoretical amount required for reaction with the cyclohexanone. Ammonia is introduced into the reaction mixture while agitating to produce at the completion of the oximation a reaction mixture having a pH of from 3 to 3.5, preferably 3.2 to precipitate the cyclohexanone oxime. The precipitate is separated from the ammonium sulfate mother liquor or brine by a centrifugal separator or by filtration.
Although the process of U.S. Pat. No. 3,070,627 is considered a major contribution to this art, research efforts have been continued in an effort to improve the process.
The present invention is an improvement offering advantages of very high yields and/or low production of by-products and/or improved recovery of the hydroxylamine used in excess in the oximation step of the process. This improvement consists in a combination of steps and conditions to be employed as described hereinafter.